1. Field of the Invention
This invention relates to a method for introducing a hydrocarbon into a chlorosilane under moderate conditions using inexpensive reagents.
2. Prior Art
Chlorosilanes having a high degree of alkyl substitution and derivatives thereof have found a wide variety of applications in the industry. For example, trimethylchlorosilane is not only widely used as a silylating agent, but is also effective for rendering inorganic substances hydrophobic and introducing terminal block units into organopolysiloxane chains. t-butyldimethylchlorosilane is also widely used as a silylating agent for the synthesis of intermediates of medicine. Hexamethyldisilane is also useful as a silylating agent of medicines and other products.
Since a distillation kettle for producing silanes contains methylchlorodisilanes having a high degree of chlorine substitution, it is of industrial significance to methylate the methylchlorodisilanes for conversion into disilanes having a high degree of methyl substitution.
For alkylating chloromonosilanes, Grignard reagents are conventionally used. This method has the disadvantages that metallic magnesium is expensive and a large volume of solvent must be used to detract from volumetric efficiency. In Z. Anorg. Allgem. Chem., 287273 (1956), an attempt was made to methylate chlorosilanes using methylaluminum-sesquichloride. It is known that one of the reagents, methylaluminum-sesquichloride is spontaneously ignitable in air and undergoes explosive hydrolysis in the presence of a trace amount of water. This attempt is dangerous for commercial synthesis.
Japanese Patent Application Kokai (JP-A) No. 256688/1990 discloses a method for methylating a chlorosilane by effecting a gas phase reaction between a chlorosilane and methyl chloride gas in a reactor tube at 180.degree. to 450.degree. C. This method is also dangerous in that the reaction requires a high temperature, and aluminum chloride as a by-product tends to clog the reactor tube.
Also known in the art are alkylation of chlorosilanes using alkyllithium and alkylsodium (see J. Am. Chem. Soc., 68, 1675 (1946)) and alkylation of chlorosilanes using alkylzinc (see Ann., 222, 354 (1884)). These methods are not commercially acceptable since they are inferior in safety and operation efficiency. Particularly the latter method is low in yield.
As mentioned above, the prior art methods for preparing monosilanes having a high degree of alkyl substitution suffer from problems including the use of expensive reagents, complex operation, and a high cost.
As to the methylation of chlorodisilanes, a method using Grignard reagents is also conventional as described in Kumada et al., J. Org. Chem., 21 (1956), 1264-1268. This conventional method has the disadvantages that metallic magnesium is expensive and a large volume of solvent must be used to detract from volumetric efficiency.
Japanese Patent Publication (JP-B) No. 7433/1986 proposes to carry out methylation reaction by effecting disproportionation between a chlorodisilane and tetramethylsilane in the presence of an organic aluminum compound such as ethyl aluminumsesquichloride, a silane compound containing a Si--H bond, and hydrogen chloride gas. This methylation, however, must use the organic aluminum compound, which is spontaneously ignitable and thus quite dangerous, and the tetramethylsilane which has a low boiling point and is thus inconvenient to store and handle. Because of thermodynamic equilibrium, it is essentially impossible to obtain disilanes having a high degree of methyl substitution in high yields. These facts, combined with complex operation and potential danger, render this method commercially unacceptable.
Among disilanes having a high degree of methyl substitution, hexamethyldisilane is of particular importance. In addition to the above-mentioned methods, it is also known in the prior art to prepare hexamethyldisilane through condensation of trimethylhalogenosilane using alkali metals. Methods using metallic lithium are disclosed, for example, in H. Gilman et al., J. Organometal. Chem., 13, 323 (1968); Sakurai et al., JP-A 42616/1974; D. E. Seilz et al., Synth. Commun., 9, 451 (1981); and G. Fritz et al., Z. Anorg. Allg. Chem., 473, 59 (1981). Methods using metallic sodium are disclosed, for example, in W. Sundermeyer et al., z. Anorg. U. Allgem. Chem., 310, 50 (1961); G. R. Wilson et al., J. Org. Chem., 26, 557 (1961); and M. G. Voronkov et al., Z. Obs. Khim., 26, 584 (1956). Since most of these methods use polar solvents such as tetrahydrofuran and hexamethylphosphoramide and commercially uncommon techniques such as ultrasonic techniques, they suffer from the problems of solvent recovery, inferior volumetric efficiency, and complex process. The use of alkali metals which are dangerous is also a problem in insuring industrial safety. It is further known to condense trimethylchlorosilane using magnesium (see L. Roesch et al., Z. Naturforsch. B: Anorg. Chem. Org. Chem., 31b, 281 (1976)). This method uses magnesium which is expensive and hexamethylphosphorotriamide which is recently regarded carcinogenic. This method is now unacceptable.
There is a desire to have a method for introducing hydrocarbons into chlorosilanes for producing silanes having a high degree of hydrocarbon substitution while avoiding economical and operational problems.